In brief, and in generalities rather than scientifically precise detail:

When breast tumors try to move out of the breast and invade other parts
 of the body, often the first outside-the-breast spot where they
migrate is into lymph nodes in the underarm area.

The theory is that a tumor in a particular spot is most likely to first
invade a specific lymph node (or couple of 'em) that are most closely
connected to the area where the tumor started.

But, it's hard to tell exactly which lymph node(s) that would be, as we
all have slightly different lymphatic systems.

So, the doctors inject a dye at the location of the tumor.  The dye
seeps through the lymphatic system.  The first lymph node(s) where the
dye appears are the node(s) most likely to contain cancer cells.

They're called the "sentinel node(s)" be cause they sort of stand guard
between the tumor and the rest of the lymphatic system (and body).

The doctors then remove the sentinel nodes and examine them under a
microscope.  If there are no cancer cells in them, it is unlikely that
the cancer has migrated into the lymph nodes at all.  So, they don't
remove any more lymph nodes.

If there -are- cancer cells in the sentinel node, then many doctors go
ahead & remove other lymph nodes in the area to check things further.

The older alternative to the sentinel node procedure is for the docs to
simply go straight to removing many of the lymph nodes in the underarm
area to check them all microscopically, instead of looking at just the
sentinel node.

Does that make sense?  (I'm sure that if someone else thinks this is an
inaccurate explanation, they'll correct me . . . . )

Ann T.
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Well here's my 2 cents on sentinel node.  When the dye drains to a node the
very first one that it drains to is called the sentinel node.  Or the first
node.  And that's the one they remove to check for cancer.  And if there is
no cancer in the "first node" then they believe that it has not spread.